The present invention relates to driving a solenoid valve at a low power, and more particularly to an apparatus and method for driving a solenoid valve at low power that is certifiable as intrinsically safe.
A solenoid valve is an electromechanical device commonly used to control the flow of gas or liquid by running an electric current through a solenoid, which is a magnetic coil. It is well known that a solenoid, or any electromechanical device with a magnetic coil, may be actuated at a higher voltage (power) and the voltage can be reduced to a lower voltage to “hold” the solenoid in the “on” position. The higher voltage overcomes inertia to start the movement of the solenoid, and the reduction to the lower voltage reduces power consumption in driving the solenoid.
Reducing power consumption is very desirable, particularly for circuits used to drive a solenoid valve in a process instrument sample system. However, the challenge that arises for process instrumentation is that such a circuit to drive a solenoid must use a protection technique that is suitable for the hazardous area in which it is present. Due to the difficulty of wiring to explosion-proof mechanically protected equipment, Intrinsically Safe (IS) circuits are desirable. Intrinsic safety is a protection concept deployed in sensitive or potentially explosive atmospheres. Intrinsic safety relies on equipment designed so that it is unable to release sufficient energy, by either thermal or electrical means, to contribute to conditions that are favorable for ignition of a flammable gas. There are various IS standards set forth by various certifying agencies for a system to be considered IS. Such standards include International Electrical Commission (IEC) IEC 60079-11, Factory Mutual (FM) 3610, Underwriters Laboratories (UL) UL913, etc.
Conventional circuit designs for driving a solenoid typically utilize multiple voltages, which makes such circuits difficult to certify as IS by analysis based on an IS standard. In such cases, certifying agencies must perform tests on the circuit designs using actual explosive mixtures of gases, and producing sparks with faults introduced into the circuit. Such testing is much more costly and time consuming than proving an IS design by straightforward analysis. Further, such testing leads to inefficient “trial and error” design/test cycles because it is difficult for a designer to anticipate whether the circuit design will be accepted to be IS by the certifying agency. This testing approach also makes it much more difficult to modify or iterate the design without re-test. Also, for some solenoid valves, if the inertial energy requirements to actuate the solenoid valves are high, it may not be possible for conventional circuit designs to comply to IS standards.